A Record of a Mortal is Journey to Immortality Wiki

To make a breakthrough to the Zenith Heaven early stage need 360 immortal apertures (open 252 immortal apertures in addition to the already opened 108 immortal apertures) and then need the extremely precious Profound True Essence to transform their Golden Immortal soul into a Zenith Heaven soul.

The Profound True Essence was extremely precious. It was said that one would only be able to find a sliver of it in the most extreme and pure places of the heavens and earth after passing through the astral winds of the nine heavens and entering the extremely dangerous outer space.

Advancing from the Jade Taiyi stage to the Zenith Heaven stage was not only a breakthrough in cultivation, but more importantly, one's soul would undergo a massive transformation and begin to fuse with the physical body. Henceforth, every single movement of a Zenith Heaven cultivator would possess an overwhelmingly powerful soul.

To cultivate even further at the Zenith Heaven realm one has always have to open up immortal apertures but they have to also slay Corpses, a total of three impurities in a Zenith Heaven Immortal that shall be cut from himself (from his soul) and suppressed to achieve perfection in his own being and comprehension of the law of heaven and earth, there a total of three corpses corresponding to each bottleneck, one to reach intermediate stage (840 apertures), late stage (1320 apertures) and peak stage (1800 apertures), thus it is known as slaying the Three Corpses (Evil, Good and Self Corpse)

Corpse slaying can be divided into two parts. The first step would be to sense the presence of one of the three corpses in the body. The second step would be to realize one's true nature and comprehend the will of each of the three corpses before slaying them..

When one cuts a Corpse from his body, it produces another self that represents the impurity that was removed and possessing the exact same cultivation as one self, but without the physical cultivator's body. This other self would always have the desire to replace the original cultivator to take the original body that would be a boost to their own cultivation. When one fails to suppress one of the corpses, there is an equivalent of a Zenith Heaven Immortal wanting to replace him at any moment (in total then three Zenith Heaven Immortals); suppressing the Three Corpse is the only way to get rid of them, as killing them would be the same as harming oneself.